The Federal Communications Commission and cable television testing organizations such as Cable Labs, have been evaluating digital television delivery systems in order to choose a new television "standard" which someday will replace NTSC in the United States. These systems all involve digital coding and data compression techniques, for example those utilizing the MPEG algorithms or variations thereof.
The FCC plans to test and approve an HDTV standard for terrestrial broadcasting in 1994. Although the specifics of the standard are yet to be fully tested and agreed upon, the FCC has indicated that the system will initially take the form of a so called "simulcast" approach. The new HDTV signals will have to fit into currently unused television channels (so-called "taboo" channels) and initially co-exist with conventional analog television signals without co-channel interference.
NTSC will be used hereinafter to represent one example of conventional television broadcasting. Other examples would be SECAM and PAL. Although NTSC is exemplified herein, it is not meant to be construed as a limitation and will be used herein synonomously with "conventional" to represent conventional television in general.
In 1994 the FCC will test the so-called "Grand Alliance" system, a system which was developed cooperatively by the corporate sponsors which developed the first round of individual proposals which were tested by the FCC in 1991 and 1992. This system proposes to take the best features from those systems already tested in order to present a single optimum system for FCC approval as the U.S. standard.
The Grand Alliance has already decided on a coding algorithm which will comply with the source and channel coding standards proposed by MPEG (Motion Picture Experts Group). In addition, two RF transmission schemes will be evaluated for best performance and the better of the two will be selected for inclusion in the Grand Alliance system.
The first system, which was proposed by the Advanced Television Research Consortium (ATRC), which included Grand Alliance members Philips Electronics, North America Corporation, David Sarnoff Research Laboratories and Thomson Electronics, is described in "Advanced Digital High Definition Television--System Specification", Jan. 20, 1992 which is incorporated by reference herein. The ATRC system features the use of quadrature amplitude modulation (QAM).
The second system, which was developed by Grand Alliance member Zenith Electronics, utilizes a multi-level vestigial sideband modulation approach and is discussed in U.S. Pat. Nos. 5,086,340 and 5,087,975 which are both incorporated by reference herein.
Co-channel interference between the conventional television transmission and the "simulcast" RDTV transmission could cause significant degradation in the performance of the HDTV transmission. Techniques have been proposed to alleviate this degradation in both of the proposed transmission systems.
In the Zenith approach, as described for example in "Digital Spectrum Compatible--Technical Details", submitted to WP1 by Zenith and AT&T, Sept. 23, 1991 and U.S. Pat. No. 5,121,203 which are incorporated by reference herein, a comb filter is used in the receiver to introduce nulls in the digital spectrum at the locations of the picture, color and the sound carriers. This provides a significant improvement in performance when conventional television, e.g. NTSC, is broadcast on a co-channel.
To implement such a comb filter, it is necessary that the data at the HDTV transmitter be pre-coded. This pre-coding is required to eliminate error propagation and is related to the post-comb used in the receiver, in a unique way. For example, to reduce co-channel NTSC interference, it is preferable to use a comb-filter with a delay element of 12 symbol intervals in the post-comb, although other delays can also be used as described in the '975 patent. For a delay of 12 symbols in the post-comb, it is necessary to have a delay element of exactly 12 symbols in the pre-coder as well. In general, the pre-coder for a given post-comb can be designed as explained in the references cited above.
When co-channel NTSC is not present however, the use of a post-comb at the HDTV receiver can cause a loss in error performance (e.g. 3 dB) when only additive white Gaussian noise (AWGN) is present. This is discussed in the '340 patent. Hence, an alternate path is provided at the receiver for the case when the co-channel NTSC is not present. This alternate path performs a post-coding operation, which is simply the inverse of the precoding operation at the transmitter. The path selected depends upon the measured error-rate of the periodically sent data field sync symbols at the outputs of both the post-coder and the post-comb paths. Whichever error is smallest at the end of a preset period determines whether the post-coder or post-comb is active.
A problem exists however in that when pre-coding and postcoding operations are implemented as discussed in the references, a single symbol error will cause an additional symbol error to occur which will be referred to hereinafter as "propagated symbol error". Furthermore, when the pre-coding and post-coding operations are used with a delay, for example of twelve symbol intervals, then each symbol error and its corresponding propagated error are spread twelve symbols apart.
Each symbol is created by mapping a group of bits on to a constellation. For example, two bits are required to select a symbol in a four VSB constellation as used in the Zenith system. As discussed above, a single symbol error cause a propagated symbol error to appear twelve symbols later which will result in two byte errors due to the error propagation of the single symbol error.
Since the outer RS (Reed-Soloman) decoder used in the Zenith system processes information as bytes, the resulting byte errors produces a 0.75 dB of loss in ultimate performance.
The object of the invention is to minimize the byte errors caused by a single symbol error and thus improve the performance of an HDTV television system which uses the pre-coding/post-comb method for minimizing co-channel NTSC interference.